This invention relates to infrared detection devices for detecting infrared radiation in a given wavelength range (for example 8 to 14 .mu.m wavelength) and comprising a flexible film having a temperature-dependent characteristic. The film may have, for example, pyroelectric and/or ferroelectric properties, or it may comprise a liquid-crystal material. Such a device in accordance with the present invention may be used in, for example, a thermal-radiation sensing system possibly for intruder detection purposes or in a comparatively inexpensive infrared camera or other thermal-imaging equipment.
One problem associated with such infrared radiation detection devices is ensuring sufficient absorption of incident radiation at the wavelengths of interest. Pyroelectric and/or ferroelectric materials suitable for detector elements (and also liquid-crystals) often have relatively low absorption over at least part of the wavelength range for which they are desired to operate. In order to reduce particularly its thermal capacitance as well as its thermal conductance to the surroundings, it is desirable for the film (and any support layer) to be very thin. However, the thinner the film material, the lower will be the overall absorption of the incident radiation by the film material.
Published European patent application EP-A- 0 269 161 which corresponds to U.S. Pat. No. 4,806,763 discloses an infrared detection device for detecting infrared radiation in a given wavelength range and comprising at least one detector element having a multiple-layer structure along the path of the radiation, which multiple-layer structure comprises a flexible film having a (pyroelectric) temperature-dependent characteristic; first and second electrical conductors of the detector element are present at respective opposite first and second faces of the film; the first conductor at the first face facing the incoming radiation comprises a resistive layer providing a sufficient effective resistance per square as to absorb a substantial proportion of incident radiation in said wavelength range; and a reflector which faces the second face of the film is present behind the film to reflect radiation transmitted by the film.
In the device disclosed in EP-A- 0 269 161, the reflector is formed by the second conductor having a sufficiently low electrical resistance per square as to reflect the radiation at the second face of the film back towards the absorbing first conductor layer. The film is supported on a mount arrangement with at least most of the area of the second face of the detector element(s) free of contact with the mount arrangement and separated by a gap from a facing surface of the mount arrangement. This mount arrangement reduces thermal conduction to and from the detector element(s) and so enhances the temperature response of the film to the incident absorbed radiation.
In this known device of EP-A- 0 269 161 the radiation absorption for a polymer pyroelectric film is increased in order to improve the sensitivity of the device. This is achieved by giving the first conductor a suitable electrical resistance per square substantially to absorb the incident radiation and by selecting the thickness of the film to reduce reflection of the radiation. Reflection is a minimum at wavelengths for which the optical thickness of the film (i.e. the product of its physical thickness and its refractive index) is an odd number of quarter wavelengths. The greatest bandwidth over which reflection is reduced is obtained by making the optical thickness equal to substantially one quarter of a selected wavelength. This wavelength is suitably selected bearing in mind that reflection increases fairly rapidly to a maximum as the wavelength of the incident radiation decreases from the selected value to half the selected value but increases much less rapidly as the wavelength increases from the selected value, and taking into account the spectral absorption characteristics of the material of the film and, to some extent, preceding optical elements such as a window of the device. When the detector device is intended to be used for detecting infrared radiation characteristic of a human body (for example in an intruder detection system) or other room-temperature objects, the wavelength range of interest is approximately 5 to 15 .mu.m (micrometers), and it is chosen to maximise absorption of radiation by the resistive conductor layer at a wavelength of 8 .mu.m. In this case, the optical thickness of the film is selected to be substantially one quarter of this wavelength, i.e. to be approximately 2 .mu.m.
The conductor at the first face of the device film of EP-A- 0 269 161 is immediately preceded by a space and is chosen to have a resistance per square value of approximately 377 ohms (this being the characteristic impedance of free space) to optimise absorption of radiation. The conductor layer at the second face of the pyroelectric film is reflective with a resistance per square adjacent the film of the order of 5 ohms or less.
Published United Kingdom patent application GB-A-2 173 038 which corresponds to U.S. Pat. No. 4,704,534 and European patent application EP-A- 0 272 731 which corresponds to U.S. Pat. No. 4,806,762 disclose other infrared detection device structures in which the first conductor is given a suitable resistance per square to absorb the incident radiation and is preceded by one or more dielectric support layers of quarter-wavelength thickness. The whole contents of EP-A- 0 269 161, EP-A- 0 272 731 and GB-A-2 173 038 are hereby incorporated herein as reference material. The structures disclosed in these three published patent applications provide very efficient absorption of the infrared radiation to be detected. In the specific examples given, the optical thickness of the pyroelectric material is about 2 .mu.m (a quarter of a wavelength) in EP-A- 0 269 161 and an order of magnitude larger in EP-A- 0 272 731 and GB-A-2 173 038.
In order to produce infrared detector elements with low thermal mass, short response time, wide spectral response, and good sensitivity, there is considerable interest in the use of Langmuir-Blodgett films with ferroelectric and/or pyroelectric properties and of liquid-crystal films. Unlike conventional ferroelectric and/or pyroelectric materials, Langmuir-Blodgett films do not require poling. Published PCT application WO-A-87/00347 discloses particular examples of Langmuir-Blodgett films with pyroelectric and ferroelectric properties, and published United Kingdom patent application GB-A-2 163 566 discloses some examples of liquid-crystal films used for infrared detection devices. The whole contents of both WO-A-87/00347 and GB-A-2 163 566 are hereby incorporated herein as reference material. In both these cases, the optical thickness of the films is much smaller than a quarter of a wavelength in the wavelength range being detected. Thus, for example, in typical cases the Langmuir-Blodgett films and liquid-crystal films may have a physical thickness of between 0.05 .mu.m and 0.5 .mu.m, and so their optical thickness may be an order of magnitude thinner than the pyroelectric polymer films in EP-A-0 269 161 and two orders of magnitude thinner than the pyroelectric elements in GB-A-2 173 038 and EP-A- 0 272 731.